This invention relates to an acoustical ceiling tile having an improved sound absorption value. More particularly, this invention relates to a dual layer acoustical ceiling tile having a low or no mineral wool base mat layer and a high mineral wool overlay surface layer which provides improved sound absorption values with or without perforating or fissuring the tile. The invention also relates to a dual layer acoustical tile which is manufactured using a high speed, water-felting process. A pattern can be applied before drying the tile (wet end embossing), or the pattern can be formed in the tile after the drying.
The water-felting of dilute aqueous dispersions of mineral wool and lightweight aggregate is a commercial process for manufacturing acoustical ceiling tile. In this process, a dispersion of mineral wool, lightweight aggregate, binder and other ingredients as desired or necessary is flowed onto a moving foraminous support wire, such as that of a Fourdrinier or Oliver mat forming machine, for dewatering. The dispersion is first dewatered by gravity and then vacuum suction is applied. After vacuum dewatering, the wet mat is dried in heated convection drying ovens, and the dried mat is cut to the desired panel or tile dimensions. If desired, the panels or tiles can be top coated with paint.
Acoustical ceiling tiles can also be made by a wet pulp molding or cast process such as described in U.S. Pat. No. 1,769,519. In accordance with this process, a molding composition comprising granulated mineral wool fibers, fillers, colorants and a binder (e.g. starch gel), is prepared for molding or casting the tile. The composition is placed upon suitable trays which have been covered with paper or a metallic foil and then the composition is screeded to a desired thickness with a screed bar or roller. A decorative surface, such as elongated fissures, may be provided by the screed bar or roller. The trays filled with the mineral wool composition are then placed in an oven to dry or cure.
In U.S. Pat. No. 5,250,153, issued Oct. 5, 1993, a process is disclosed for making mineral wool panels on a foraminous support wire by forming a dilute aqueous dispersion of mineral wool fibers and/or aggregate and an anionically stabilized latex binder. The binder is deposited onto the mineral wool fibers by adding a small amount of a cationic flocculant. Substantially all of the binder latex becomes coupled to the mineral wool fibers. The wet mat can be dried quickly by passing heated air through the mat that is capable of maintaining its structure.
In my U.S. Pat. No. 5,558,710, issued Sep. 24, 1996, I disclose a gypsum/cellulosic fiber composition that can replace all or a portion of the mineral wool normally present in acoustical ceiling tiles. The gypsum/cellulosic fiber composition is combined with a lightweight aggregate material and a binder to form a composition that is used in a water-felting process to manufacture acoustical ceiling tiles.
As disclosed in Example 1 of U.S. Pat. No. 5,558,710, a water-felting process was used to make the acoustical tiles. The feed slurry during mat formation was held at 4% solids, and this 4% solids consistency was also used in making the control tile. The control tile, using 100% mineral fiber (i.e. no gypsum/wood fiber) had the following formulation:
Samples of the control tile were tested for NRC (noise reduction coefficient) using the Impedance tube method. The samples were not perforated, fissured or painted. The control tiles had an average NRC value of only 0.434.
In general, acoustical tiles made using a water-felting process have a hard surface that does not have good sound absorption properties. The sound absorption is substantially improved by fissuring and/or perforating the surface that increases the NRC value. However, many purchasers prefer a smooth, unperforated acoustical ceiling tile for its aesthetic appearance.
As disclosed in U.S. Pat. No. 5,250,153, acoustical ceiling tiles having an average NRC equivalent to commercially available cast ceiling tiles can be made by using an anionically stabilized latex binder and a cationic flocculant to couple the latex binder onto the mineral fiber materials. In the acoustical tiles made by this process, the mineral fibers constitute about 50% or more of the total dry solids, preferably from about 60 to about 95 weight % of the acoustical panel. However the tiles made by this process are quite soft compared to the water-felted tiles having a starch binder. In addition, the tiles made with a latex binder have lower structural strength and are made in thicknesses of at least about xc2xd inch and frequently have a woven scrim applied thereto to increase strength. These acoustical tiles do have smooth surfaces and higher NRC values resulting from the higher mineral wool content.
Mineral wool acoustical tiles are porous which is necessary to provide good sound absorption. The prior art (U.S. Pat. Nos. 3,498,404, 5,013,405 and 5,047,120) also discloses that mineral fillers, such as expanded perlite, may be incorporated into the composition to improve sound absorbing properties.
It is an object of this invention to provide an acoustical tile having a dual layer of acoustical materials both of which contain mineral fibers or having a base mat with no mineral fibers.
It is another object of this invention to provide a water-felted base mat having a relatively low mineral fiber content or no mineral fibers and a surface layer having a high mineral fiber content to form an acoustical tile with improved sound absorbing properties.
It is a further object of this invention to provide a dual layer acoustical ceiling tile having a smooth, unperforated surface and also good sound absorbing properties.
It is a still further object of this invention to provide a dual layer acoustical ceiling tile having a sound absorption value (NRC) of at least about 0.50.
These and other objects will be apparent to persons skilled in the art in view of the description that follows.
It has been discovered that a dual layer acoustical ceiling tile having an improved sound absorption value (NRC) can be made in a water-felting process wherein a base mat layer has a relatively low mineral fiber content, and a surface layer having a high mineral fiber content is overlaid onto the base mat. The base mat layer is made from a low mineral fiber content or no mineral fiber material which has relatively low NRC values unless its surface is perforated and/or fissured. The mineral fiber-rich surface layer that has a thickness of about xc2xc inch or less also has a relatively low NRC value at such thickness. It was discovered that these two low NRC value materials could be combined to provide a dual layer ceiling tile having a high NRC value.
The acoustical ceiling tiles of this invention are based on the discovery that two acoustical materials having relatively low NRC values can be combined to form a dual layer acoustical ceiling tile having excellent sound absorption values (NRC). These ceiling tiles are made using a water-felting process to form both the base mat layer and the fiber-rich surface layer. In carrying out the process, two head boxes are used to feed the acoustical materials to the production line.
One head box feeds the base mat material, having a relatively low mineral fiber content (less than about 50% by weight of mineral fiber) or it may contain no mineral fiber, to a moving foraminous support wire, such as that of a Fourdrinier or Oliver mat forming machine for dewatering. After water is removed through the support wire by gravity, additional water can be removed by applying a vacuum to the wet base mat, but depending upon the consistency of the base mat material in the head box, the line speed and other considerations, it may not be necessary to use vacuum for dewatering purposes prior to depositing the fiber-rich overlay material onto the base mat. The base mat material consists essentially of mineral wool fibers, expanded perlite, cellulose fiber, starch binder and gypsum which can be present, preferably, in the following amounts, and having at least about 30% by weight of expanded perlite:
After the initial dewatering of the base mat material on the wire support, the still wet base mat may be passed under a press roller to compress the mat, removing more water and establishing the thickness of the wet base mat. In general, the thickness of the wet base mat just prior to depositing the fiber-rich surface layer may range from about 1 inch to about 2.5 inches. It is preferred that the completely dried base mat have a thickness ranging from about 0.25 inch to about 0.625 inch.
If desired or necessary to strengthen the dual layer ceiling tile, a fiberglass scrim can be placed on the wet base mat prior to depositing the fiber-rich surface layer. The fiberglass scrim can be either woven or non-woven. If a fiberglass scrim is used, it is generally preferred that it be placed between the base mat material and the fiber-rich surface layer, however, if desired, the scrim can be placed on top of the fiber-rich surface layer or in contact with the back of the base mat material, in which case, the base mat slurry from the head box would be deposited on the scrim.
The fiber-rich surface layer consists essentially of mineral wool fibers, gypsum, clay filler, latex binder, starch binder and flocculant to deposit the latex binder on the mineral wool fibers as disclosed in U.S. Pat. No. 5,250,153. These ingredients may be present, preferably, in the following amounts:
The fiber-rich surface material is prepared in accordance with the method disclosed in U.S. Pat. No. 5,250,153 wherein an anionically stabilized latex binder is deposited on or coupled to the mineral fibers by adding a small amount of a flocculant such as a cationic polyacrylamide to the slurry. In accordance with this invention, the fiber-rich slurry contains a very large amount of mineral wool fibers (at least about 75% by weight) and little or no expanded perlite. The fiber-rich material is deposited on the base mat from a second headbox to form a dual layer material which is dewatered by applying a vacuum to the wet dual layer material and also by passing the wet dual layer material under a press roll. The press roll helps to remove some of the water. The fiber-rich surface is textured and the thickness of the dual layer material is established under the pattern/texture roll. The dual layer material is subsequently passed to an oven to complete the drying process and to cure the starch and latex binders.
When completely dried and cut into ceiling tiles, the dual layer material has a smooth or textured surface that is rich in mineral wool fibers and unperforated. In general, it is preferred that the dried dual layer ceiling tiles have a total thickness ranging from about 0.5 inch to about 1 inch, with the thickness of the fiber-rich surface layer ranging from about 0.125 inch to about 0.5 inch. The thickness of the wool-rich surface layer can be increased from about 0.5 inch to about 0.625 inch to provide higher NRC values.
Prior to drying the dual layer material in an oven, it is preferred to apply a xe2x80x9cwet end coatingxe2x80x9d to the mineral fiber-rich surface, which is smooth and unperforated. One or more coats of paint may be spray applied. It has been found that the application of paint actually increases the NRC value, because the unpainted surface tends to reflect the sound and therefore has a lower NRC (noise reduction coefficient).
Other ingredients may also be present in either the base mat or the fiber-rich surface layer or both layers. Examples of such ingredients include dyes, pigments, inorganic fillers, antioxidants, surfactants, water repellents, fire retardants and the like.
As noted above, gypsum (calcium sulfate dihydrate) is preferably present in both layers. The gypsum is soluble in the aqueous slurry comprising both the base mat and the fiber-rich layer feed material. The solubility of the gypsum in the processing slurry enables the gypsum to function as a flocculant in the slurry formulation. The flocculating function provides uniform distribution of fine particles (e.g. clay, gypsum, perlite and starch) present in the formulation during mixing. This flocculating function helps to prevent the fine and high density particles from migrating to the bottom of the mat. In addition, the gypsum helps to disperse the mineral wool fibers in the aqueous slurry.
A starch binder is also present in both the base mat and the fiber-rich surface layer. It is preferred to use the starch in the form of a gel which is prepared by dispersing starch particles in water and heating the slurry until the starch is fully cooked and the slurry thickens to a viscous gel. If the binder is corn starch, cooking temperatures may range from about 180xc2x0 F. (82xc2x0 C.) to about 195xc2x0 F. (90xc2x0 C.). It should be noted that starch may also be used as a binder without pre-cooking the starch to form a gel. In addition, the starch can be used in a pre-gelatinized form which is converted to a gel merely by adding it to water, without the need to cook it.